Thomson was born in 1832, and travelled to India at an early age. In 1853 he joined the 53rd Bengal Native Infantry – a regiment with Indian troops and white officers - as a subaltern, and in early 1857 moved with the regiment to Cawnpore. During the Indian Mutiny, the regiment was caught up in the Siege of Cawnpore; Thomson led a small party of men in the attempt to escape, and was one of only four male survivors from the garrison.[1]

He was invalided home, heavily wounded, and promoted to brevet-major. On returning to duty he was given a civilian post as political agent at Manipur, and later appointed Governor-General's agent for Wajid Ali Shah, the former King of Oudh. He retired in 1885, with the Army rank of major-general, and was promoted to full general in 1894.[1]